In recent years, there has been considerable interest in the production of glass fibers. Due to the tremendous usages of glass fibers, the interest has been particularly focused on increasing the production of individual fiber drawing stations. In the production of fibers, molten glass is typically passed through nozzles or orifices in a bushing to create individual fibers. In addition to the problem of increased production, the apparatus used in such processes is typically quite expensive as it often times involves the use of inordinate amounts of platinum, complex orifices, high pressure generating equipment, pressure resistant bushings, etc.
To increase the yield of the drawing stations, the obvious but difficult approach is to increase the number orifices per bushing through which the molten glass is directed and from which the individual fibers are formed. A few years ago, a standard bushing produced 204 fibers. After considerable expense and research, the capacity of the bushing has been increased to 2,000 orfices and, through a recent breakthrough in the art, arcuate bushings have been developed which are used in conjunction with high pressure glass and are capable of handling 6,000 orifices. While the total number of bushings per drawing station could, of course, be increased to raise the production of the station, such an approach would be self-limiting, increase the bulk of the station, complicate the operation as well as increase the costs and result in the decrease of uniformity of the individual fibers over the entire station. Therefore, it is the individual bushing orifice function and capacity which is a substantial limiting factor in the production of a high volume of quality fibers from a drawing station.
A single bushing 10-inch square, incorporated in the method and apparatus hereinafter to be described, can support 102,400 orifices. This represents 51.2 times the number of glass fibers which can be produced by the best of the current standard systems and 17 times more than the aforementioned arcuate bushing system are capable of producing. In spite of this tremendous gain, the present invention is remarkably simple both in concept and construction. It utilizes the simplest of bushings containing a minimum amount of platinum, the base of which is formed into a generally flat, thin horizontal orifice plate with just plain holes therein.
Plain holes in flat, wettable alloy plates have been utilized in the past, however, they have been quite limited with respect to the proximity of adjacent orifices. If the hole edges are closer than about one-half-inch apart, the glass will often times creep through capillary action along the underside of the plate to join and break an adjacent fiber. Such flow will continue in an ever-widening cycle resulting in a flooding of the entire bushing. The underside of the plate will become coated with a single, useless glob of glass. With non-wettable alloys, where the wetting angle is 55.degree. or more, plain holes can, of course, be closer together, but the spacing nevertheless is limited to the diameter of the drops passing therethrough. Even with the best of non-wettable alloys, should two drops touch, they will immediately flow together to form a single larger drop and be forced to wet the plate. This wetting cycle will continue between the remaining orifices resulting in a totally unusable bushing.
There are other systems which employ plain holes in a flat bushing wherein a burnable gas is directed at the bushing and the forming cones. Upon contact with the high temperature of the issuing glass and of the bushing, the gas decomposes to deposit a carbon coating on the plate and glass. Due to the poor wetting characteristics of glass on carbon and graphite, the carbon which coats the glass drops allows the drops to be pushed together without coalescing. Despite the increased proximity within which the orifices may be placed in such a system, the overall production of the system is still quite limited due in part to the degrading effect of carbon on the glass. Moreover, such systems are unreliable because once flooding occurs, it is very difficult to effect separation thereafter. Other systems actually employ carbon inserts coaxially disposed around each orifice. Such systems occupy a greater amount of space and the carbon must be employed in an inert atmosphere. This, of course, further increases the cost of such a system and the inert gas along with the exposure of the carbon may have a damaging effect on the surface of glass fibers which desireably should be disposed in an oxidizing atmosphere. Such systems, therefore, do not present a satisfactory solution to the problem of increasing production.
U.S. Pat. No. 3,573,014 discloses a method and apparatus for producing fibers from glass which incorporates an arcuate bushing of the type referred to above. The system disclosed therein utilizes pressure on the glass substantially above a nominal pressure head to cause separation and prevent flooding of the bushing. To withstand such pressures, the bushing must have a pressure resistant configuration. This configuration is provided by the arcuate bushing. In such a system, separation is caused by what was originally termed the shower head effect wherein the molten glass is forced at high pressure through the apertures in the bushing and the formed jet overwhelms any thin film on the bushing and in fact sucks the surrounding area dry thereby preventing flooding and thereafter maintaining individual separated glass fibers. This high pressure, as well as requiring additional equipment for its generation, dictates the configuration of the bushing used in such a system which in turn limits the number of fibers which can be produced by such a system.
In providing an economical system with increased fiber production, in addition to developing a bushing with a greater number of orifices therein, it is necessary first to cause separation and thereafter to maintain the separation of each of the formed fibers. Glass has an unusually high surface tension and, therefore, a droplet is constrained to a generally spherical configuration. To distort the drop into a fiber forming cone requires the application of stress. As molten glass passes through an orifice and is forced to form into a fiber, the base of the fiber assumes the shape of a fiber forming, asymptote-like cone. As long as there is sufficient stress to maintain the geometry of the cone, i.e., to cause some concavity, an equilibrium between the fiber and its glass source will prevail. However, such fibers cannot be sustained without exercising considerable control over the asymptotic geometry of the feeding cone.
There are several means for cooling glass fibers. The standard means comprises one or two rows of orifices sandwiched between fins which in turn are cooled by liquid or air. Variations include several hundred individual streams of air, each one of which cools a single fiber base or row of fibers either through holes in fins or by air piped through hundreds of hyperdermic needle-like tubes to direct air at or in between each fiber. Such fins are sometimes perforated to cause an overall oozing of air streams to impinge at or near the fiber roots. Such systems either cross the fibers at 90.degree. with respect to their axis or blow in a downward direction. An example to the contrary is found in U.S. Pat. No. 3,695,858 which apparently incorporates a pair of air jets for each formed filament, i.e., 20 air jets for each 10 fibers. Each of the air jet orifices are located only a few thousandths of an inch from each fiber and are downwardly and upwardly directed at 45.degree.. The pairs of jets work in unison ostensibly to cause a controlled turbulence to cool a row two tips wide within a hooded enclosure. In such systems, the physical positioning of the air orifices is extremely critical and necessarily complex.
Such systems have several problems which the hereinafter described system eliminates. Jets, fins and hoods must be located adjacent to both bushing and orifices which in turn requires very carefully positioning and position maintenance. They absorb considerable energy in cooling the bushing, occupy valuable space on the bushing and, therefore, substantially decrease the number of orifices which can be employed. In addition, these jets, fins and hoods become a recepticle for condensates from the molten glass which cause fiber breaking "flys" and which degrades the cooling efficiency and necessitates frequent cleaning. Finally, such systems have a most serious weakness in that they are limited to cooling but a few rows of orifices. When air blows across several rows of orifices at 45.degree. or less, a primary and contiguous layer hugs the plate in a laminar flow and blankets the plate to prevent additionally directed air from penetrating the blanket to cool the cones. The air tends to laminate to create successive layers which quickly become too deep to have any cooling effect on the very short fiber forming cones. Consequently, the air at the cone level over cools the first rows of fiber and under cools the successive rows. For this reason, air directed from 90.degree. to 45.degree. to the fiber axis is quite limited in the width of its quenching effect.
To constrain the cone to its fiber forming shape by cooling is not difficult with a single fiber because of 360.degree. cooling nor is cooling difficult with any number of rows of fibers one or two fibers wide. However, when there is a press of thousands of orifices tightly grouped together in a single flat bushing, a continuous and identical geometrical constraint on each of these thousands of cones presents a substantial cooling problem. Should just one of the thousands of forming cones lose its shape, the force of wetting would at once dominate and the glass would creep to an adjacent cone which would then break, the cycle of breakage and wetting continuing thereafter in an ever-widening ring until the bushing would become flooded. Therefore, in addition to providing an economical system for increased glass fiber production which incorporates a bushing having an increased number of orifices therein, it is necessary to provide a temperature control for the creation and maintenance of the asymptotic geometry of the fiber forming cones.